The term "Metallocene" as used herein refers to a derivative of cyclopentadienylidene which is a metal derivative containing at least one cyclopentadienyl component which is bonded to a transition metal. The transition metal is selected from Groups IVB, VB, and VIB, preferably IVB and VIB. Examples include titanium, zirconium, hafnium, chromium, and vanadium. A number of metallocenes have been found to be useful for the polymerization of olefins. Generally, the more preferred catalysts are metallocenes of Zr, Hf, or Ti.
Generally, in order to obtain the highest activity from metallocene catalysts, it has been necessary to use them with an organoaluminoxane cocatalyst, such as methylaluminoxane. This resulting catalyst system is generally referred to as a homogenous catalyst system since at least part of the metallocene or the organoaluminoxane is in solution in the polymerization media. These homogenous catalysts systems have the disadvantage that when they are used under slurry polymerization conditions, they produce polymer which sticks to reactor walls during the polymerization process and/or polymer having small particle size and low bulk density which limits the commercial utility.
Some attempts to overcome the disadvantages of the homogenous metallocene catalyst systems are disclosed in U.S. Pat. Nos. 5,240,894; 4,871,705; and 5,106,804. Typically, these procedures have involved the prepolymerization of the metallocene aluminoxane catalyst system either in the presence of or in the absence of a support. An evaluation of these techniques has revealed that there is still room for improvement, particularly when the catalyst is one which is to be used in a slurry type polymerization where the object is to produce a slurry of insoluble particles of the end product polymer rather than a solution of polymer which could result in fouling of the reactor. In the operation of a slurry polymerization in a continuous loop reactor it is extremely important for efficient operations to limit polymer fouling of the internal surfaces of the reactor. The term "fouling" as used herein refers to polymer buildup on the surfaces inside the reactor.
U.S. Pat. No. 5,498,581 discloses a technique for producing insoluble polymerization catalysts by combining a metallocene having polymerizable groups with an aluminoxane and a particulate solid and then prepolymerizing. PCT /US96/13000, the disclosure of which is incorporated herein by reference, and a commonly owned copending application disclose that at least one such catalyst is capable of producing polyethylenes having ethyl branches which are uniformly distributed at the intra and intermolecular level without using any comonomer.
An object of the present invention is to provide a method for producing an insoluble polymerization catalyst that is capable of producing even more ethyl branches without the addition of comonomer. Another object of the present invention is to provide a process for producing polyethylene having unusually high levels of ethyl branches without the use of a comonomer. Still another object of the present invention is to provide polyethylenes that are particularly useful for making films of unusual clarity at a given density, said polyethylenes having a very narrow molecular weight distribution and no detectable amounts of long chain branching.
Another object of the present invention is to provide polyethylene having ethyl branches wherein the level of ethyl branches is substantially the same regardless of the molecular weight of the polymer molecules.
Another object of the present invention is to provide certain types of polyethylene having ethyl branches wherein the molar percent of ethyl branches is substantially the same for molecular fractions across the molecular weight distribution of the polymer. One type of such polyethylene is substantially free of any other type branches. In a preferred embodiment the polyethylene is substantially free of branches having 6 or more carbon atoms.
Another object of the present invention is to provide certain polyethylene having significant amounts of terminal vinyl groups, a very narrow molecular weight distribution, and a lower density than one would normally expect for the amount of branching due to comonomer insertions in the polymer chains.
Another object of this invention is to provide certain narrow molecular weight polyethylenes having an unusually low density for the amount of short chain branching at a given molecular weight. Still another object is to provide polyethylene films having a desirable combination of properties.